Flat panel, color displays for displaying information, including images, text, and graphics are widely used. These displays may employ any number of known technologies, including liquid crystal light modulators, plasma emission, electro-luminescence (including organic light-emitting diodes), and field emission. Such displays include entertainment devices such as televisions, monitors for interacting with computers, and displays employed in hand-held electronic devices such as cell phones, game consoles, and personal digital assistants. In these displays, the resolution of the display is always a critical element in the performance and usefulness of the display. The resolution of the display specifies the quantity of information that can be usefully shown on the display and the quantity of information directly impacts the usefulness of the electronic devices that employ the display.
However, the term “resolution” is often used or misused to represent any number of quantities. Common misuses of the term include referring to the number of light-emitting elements or to the number of full-color groupings of light-emitting elements (typically referred to as pixels) as the “resolution” of the display. This number of light-emitting elements is more appropriately referred to as the addressability of the display. Within this document, we will use the term “addressability” to refer to the number of independently-addressable light-emitting elements per unit area of the display device. A more appropriate definition of resolution is to define the size of the smallest element that can be displayed with fidelity on the display. One method of measuring this quantity is to display the narrowest possible, neutral (e.g., white) horizontal or vertical line on a display and to measure the width of this line or to display an alternating array of neutral and black lines on a display and to measure the period of this alternating pattern. Note that using these definitions, as the number of light-emitting elements increases within a given display area, the addressability of the display will increase while the resolution, using this definition, generally decreases. Therefore, counter to the common use of the term “resolution”, the quality of the display is generally improved as the resolution becomes finer in pitch or smaller.
Addressability in most flat-panel displays, especially active-matrix displays, is limited by the need to provide signal busses and electronic control elements in the display. Further in many flat panel displays, including Liquid Crystal Displays (LCDs) and bottom-emitting Electro-Luminescent (EL) displays, the electronic control elements are required to share the area that is required for light emission or transmission. In these technologies, the more such busses and control elements that are needed, the less area in the display is available for light emission. Depending upon the technology, reduction of the area available for light emission can reduce the efficiency of light output, as is the case for LCDs, or reduce the brightness and/or lifetime of the display device, as is the case for EL displays. Regardless of whether the area required for patterning busses and control elements competes with the light-emitting area of the display, the decrease in buss and control element size that occur with increases in addressability for a given display generally require more accurate, and therefore more complex, manufacturing processes and can result in greater number of defective panels, decreasing yield rate and increasing the cost of marketable displays. Therefore, from a cost and manufacturing complexity point of view, it is generally advantageous to be able to provide a display with lower addressability. This desire is, of course, in conflict with the need to provide higher apparent resolution. Therefore, it would be desirable to provide a display that has relatively low addressability but that also provides high apparent resolution.
It has been known for many years that the human eye is more sensitive to the spatial frequency of luminance in a scene than to color. In fact, current understanding of the visual system includes the fact that processing is performed within or near the retina of the human eye that converts the signal that is generated by the photoreceptors into a luminance signal, a red/green difference signal and a blue/yellow difference signal. Each of these three signals have a different resolution with the luminance channel having the highest spatial frequency cutoff followed by the red/green spatial frequency cutoff and finally the blue/yellow spatial frequency cutoff. In fact, the cutoff for the luminance channel is nearly twice the spatial frequency cutoff for the red/green difference signal and nearly four times the spatial frequency cutoff of the blue/yellow difference signal.
This difference in sensitivity is well appreciated within the imaging industry and has been employed to provide display devices with high apparent resolution for a reduced addressability. In one example, Takashi et al. in U.S. Pat. No. 5,113,274, entitled “Matrix-type color liquid crystal display device”, proposed the use of displays having two green for every red and blue light-emitting element. While such an array of light-emitting elements can perform well for displays with a very high addressability, it is important that the red light-emitting elements typically provide approximately 30 percent of the luminance. Therefore, under certain conditions, such as when displaying flat fields of red, it is possible to see artifacts (e.g., a red and black checkerboard pattern in areas that are intended to be perceived as a flat field red) that occur because of the scarcity of the red light-emitting elements within the array. Therefore, it is important to understand that in displays it is not only the size or the frequency of light-emitting elements that are important to understand the quality of the display device but also the space between the light-emitting elements. In fact, anytime that the distance between any two light-emitting elements of the same color subtends a visual angle greater than 1 minute of arc, it will be possible to see a checkerboard pattern when attempting to display a flat field of color.
It may be additionally desirable to include additional high luminance light-emitting elements. For example, within the field of Organic Light Emitting Diodes (OLEDs), it is known to introduce more than three light-emitting elements where the additional light-emitting elements have higher luminance efficiency, resulting in a display having higher luminance efficiency. Such displays have been discussed by Miller et al. in U.S. Patent Application Publication 2004/0113875, entitled “Color OLED display with improved power efficiency”. When applying four or more different colors of subpixels it is then further known to utilize patterns of light-emitting elements having a higher addressability of high luminance white and green light-emitting elements than arrays of low luminance red and blue light-emitting elements as discussed by Miller et al. in U.S. Patent Application 2005/0270444, entitled “Color display with enhanced pixel pattern”. Unfortunately, such an arrangement of light-emitting elements can result in the same undesirable checkerboard pattern in the color channels with lower addressability.
It is also known to provide displays having more than one color of high luminance light-emitting element and to use each of these high luminance light-emitting elements to create the high frequency luminance channel. For example, U.S. Patent Application 2005/0225574 and U.S. Patent Application 2005/0225575, each entitled “Novel subpixel layouts and arrangements for high brightness displays” provide various arrangements of light-emitting elements having two colors of high luminance light-emitting elements, such as the white and green light-emitting elements, and to arrange these light-emitting elements such that each row in the arrangement contains all colors of light-emitting elements, making it possible to produce a line of any color using only one row of light-emitting elements. Similarly, every pair of columns within the arrangement discussed within this disclosure contains all colors of light-emitting elements within the display, making it possible to produce a line of any color using only two columns of light-emitting elements. Therefore, when the LCD is driven correctly, it can be argued that the vertical resolution of the device is equal to the inverse of the height of one row of light-emitting elements and the horizontal resolution of the device is equal to the inverse of the width of two columns of light-emitting elements, even though it realistically requires more light-emitting elements than the two light-emitting elements at the intersection of such horizontal and vertical lines to produce a full-color image. However, since each pair of light-emitting elements at the junction of such horizontal and vertical lines contains one high luminance (i.e., white or green) light-emitting element, each pair of light-emitting elements provides a relatively accurate luminance signal within each pair of light-emitting elements, providing a high-resolution luminance signal. It is important to note that in arrangements of light-emitting elements such as these, as well as those discussed by U.S. Pat. No. 5,113,274, the high-luminance light-emitting elements can provide a luminance image with higher addressability than the addressability of any individual color of light-emitting element. As was the case with Takashi and Miller, displays utilizing this pixel pattern will exhibit a checkerboard pattern when a flat field, single color luminance pattern is input.
Although the reduced addressability that can be attained using pixel patterns such as U.S. Pat. No. 5,113,274, U.S. Patent Application 2005/0270444, U.S. Patent Application 2005/0225574 or U.S. Patent Application 2005/0225575 generally reduce the complexity of manufacturing the final display, these patterns also lack uniformity when displaying flat fields of color for any display in which the gap between any two color subpixels of any one color subtends an angle greater than 1 minute of arc on the user's retina. This artifact limits the use of such patterns to displays with an addressability of around 300 full color pixels per inch or greater. Displays with lower resolution will provide objectionable levels of the checkerboard artifact when viewed from some typical viewing distance. This is particularly troubling when attempting to apply these techniques in larger displays which are generally designed to have a lower addressability because they are typically viewed from a larger viewing distance. However because these displays can be viewed from near viewing distances and often are viewed from near viewing distances by individuals making purchasing decisions on show room floors, the artifacts that occur in images generated on such arrangements of light-emitting elements makes the use of such pixel patterns on larger displays impractical.
Artifact reduction using arrangements of light-emitting elements such as the “RGB delta” pattern has been taught, for example by Noguchi et al. in U.S. Pat. No. 4,969,718, that are enabled by splitting the subpixel electrodes into equal halves. However in this case the split is done solely to solve electrical problems associated with the RGB delta pattern, and the split electrodes drive identical colors and remain juxtaposed.
It is also known in the art to correct for image degradation (e.g., avoid flicker in LCD displays) by localizing the degradation on dark-colored, or low luminance subpixels, as taught in U.S. Patent Application 2005/0083277A1. It is taught therein that successive pairs of blue columns may share the same column driver through an interconnect, however the row selection mechanisms are independent, and the TFT's of the blue subpixels are remapped to avoid sharing of exact data values.
There is therefore a need to provide an enhanced arrangement of light-emitting elements, such as the ones described within this background, that require a minimum number of drive circuits and that enable the use of even lower addressabilities on full color displays. Specifically, it is desired to provide such an enhanced arrangement of light-emitting elements in displays having an addressability of less than 300 pixels per inch without creating the perception of non-uniformity within areas of an image that are intended to have a uniform color.